(1) Field of the Invention
The present invention relates generally to the fields of organic chemistry and optically anisotropic coatings. More specifically, the present invention relates to the synthesis of heterocyclic sulfoderivative compounds and the manufacture of optically anisotropic coatings based on these compounds.
(2) Description of Related Art
Modern technological progress requires development of optical elements based on new materials possessing specific, controllable properties. In particular, a necessary element in modern visual display systems is an optically anisotropic film with an optimum combination of optical and other characteristics for a particular device.
Various polymeric materials are known in the prior art for use in the manufacturing of optically anisotropic films. Films based on these polymeric materials acquire anisotropic optical properties through uniaxial extension and modification with organic dyes or iodine. In most applications, the base polymer is polyvinyl alcohol (PVA). Such films are described in greater detail in the monograph Liquid Crystals: Applications and Uses, B. Bahadur (ed.), World Scientific, Singapore-New York (1990), Vol. 1, p. 101. However, the low thermal stability of PVA-based films limits their application. It is desirable to find new materials and develop methods for the synthesis of optically anisotropic films possessing improved characteristics, in particular, higher heat resistance, more convenient synthesis, and better film-forming properties.
Organic dichroic dyes are a new class of materials gaining prominence in the manufacture of optically anisotropic films with high optical and working characteristics. Films based on these compounds are obtained through application of a layer of a liquid crystal (LC) aqueous dye solution, containing supramolecules composed of dye molecules, onto a substrate surface, followed by water evaporation. The resulting LC films acquire anisotropic properties either through preliminary mechanical ordering of the underlying substrate surface, as described in U.S. Pat. No. 2,553,961, or through subsequent application of external mechanical, electrical, magnetic or other orienting forces to the LC coating on the substrate, as described in U.S. Pat. Nos. 5,739,296 and 6,174,394. Basic properties of LC dye solutions are known in the prior art. However, extensive investigations into their application and the properties of related systems is a more recent development of the past decade. Recent studies have been motivated largely by industrial applications in liquid crystal displays (LCDs) and glazing. Dye supramolecules form a lyotropic liquid crystal (LLC) phase. In this phase, dye molecules generate supramolecular complexes having the form of columns—structural units of a mesophase. High ordering of dye molecules in the columns allows such mesophases to be used for obtaining oriented films characterized by a strong dichroism.
Another special property of dye molecules forming supramolecular LC mesophases is the presence of peripheral groups rendering these dyes water-soluble. The mesophases of organic dyes are characterized by specific structures, phase diagrams, optical properties, and dissolving capabilities. See J. Lydon, Chromonics, Handbook of Liquid Crystals (Wiley-VCH, Weinheim, 1998), Vol. 2B, pp. 981–1007.
By using dichroic dyes capable of forming LLC systems, it is possible to obtain films possessing a high degree of optical anisotropy. Such films exhibit the properties of E-type polarizers, which are related to peculiarities of the optical absorption of supramolecular complexes, and behave as retarders (phase-shifting devices) in the spectral regions where the absorption is insignificant. The phase-retarding properties of these anisotropic films are related to their birefringence (double refraction), that is, a difference in refractive indices measured in the direction of application of the LLC solution onto a substrate and in the perpendicular direction. Films formed from the LLC systems based on strong (light-fast) dye molecules are characterized by high thermal stability and light resistance.
The above properties of LLC systems account for the growing interest in these materials. New methods are extensively developed for obtaining films based on such organic dyes, the progress involving both optimization of the film application conditions and the search for new LLC systems. In particular, new LLC compositions for the synthesis of optically anisotropic films can be obtained by introducing modifiers, stabilizers, surfactants, and other additives to known dyes to improve characteristics of the films. See, for example, U.S. Pat. Nos. 5,739,296 and 6,174,394.
In recent years, there has been increasing demand for films possessing high optical anisotropy, characterized by improved selectivity in various wavelength ranges. Films with different positions of the absorption maximum, variable in a wide spectral range from infrared (IR) to ultraviolet (UV) regions, are needed. This has led to the development of an expanded assortment of compounds capable of forming LLC phases and films possessing the required properties. However, the number of dyes known to form stable lyotropic mesophases is still relatively small. Naturally, each new liquid-crystal dye becomes the object of thorough investigation.
Among water-soluble dichroic dyes capable of forming stable LLC phases, applicable in the manufacturing of optically anisotropic films, an important place belongs to disulfoderivatives of various organic dyes, including perylenetetracarboxylic acid (PTCA) dibenzimidazole (DBI) described in U.S. Pat. Nos. 5,739,296 and 6,174,394. PTCA dibenzimidazoles and diimides are widely used as dyes and pigments in various industries due to the high chemical, thermal, and photochemical stability of these compounds. These properties also explain the increased interest in these substances as potential materials for obtaining optically anisotropic films for LCDs and other optical devices.
The main difficulty hindering use of the above dyes is their poor solubility in water and some organic solvents. In order to provide the dyes with sufficient solubility in organic solvents, various substituents have been introduced into the molecules. Examples of such substituents are oxyethyl groups [see R. A. Cormier and B. A. Gregg, Phys. Chem. 101(51), 11004–11006 (1997)] and phenoxy groups [see H. Quante H. Y. Geerts, and K. Mullen, Chem. Mater. 6(2), 495–500 (1997)]. The solubility of perylene dyes has also been increased by amino groups [see I. K. Iverson, S. M. Casey, W. Seo, and S.-W. Tam-Chang, Langmuir 18(9), 3510–5316 (2002)] and sulfonic groups [see U.S. Pat. Nos. 5,739,296 and 6,174,394]. The best results were obtained with sulfonic groups, which provided for sufficient solubility and the formation of a stable LLC phase of perylene dyes.
The standard procedure of obtaining disulfoderivatives is as follows. To a certain volume of chlorosulfonic acid, one adds calculated amounts of PTCA DBI and oleum. Upon termination of the reaction, the mixture is colored and diluted with water. The precipitate is filtered, washed with hydrochloric acid, and dried. This yields water-soluble dibenzimidazole perylenetetracarboxydisulfonic acid, which is then dissolved in water and purified. An analysis of the system texture has shown that, beginning with a certain dye concentration, a stable hexagonal lyotropic mesophase is formed in a given temperature interval. Accordingly, a nematic phase is observed within a sufficiently narrow range of dye concentrations and temperatures. The boundaries of existence of isotropic phases, as well as two-phase transition regions, have been determined in this system.
Various dye compositions (inks) for the obtaining of polarizer films, based on PTCA DBI sulfoderivatives, have been described in patents. In particular, dyes having the following structural formula are known:
where
R=H, alkyl group, halogen or alkoxy group; and Ar is a substituted or nonsubstituted aryl radical. Such dyes, which are selective in the region of 550–600 nm, are described in U.S. Pat. No. 5,739,296.
Other dyes based on PTCA DBI have the formula:
where R1=H, 3(4)-CH3, 3(4)-C2H5, 3(4)-Cl, 3(4)-Br; and R2=4(5)-SO3H. Such dyes, which are also selective in the region of 550–600 nm, are described in SU Patent No. 1,598,430.
LC blends of PTCA DBI sulfoderivatives where various modifying additives are introduced to improve the characteristics of anisotropic films are described in U.S. Pat. Nos. 5,739,296 and 6,174,394. Indanthrone disulfoderivatives with various substituents are described in U.S. Pat. Nos. 5,739,296 and 6,174,394. Compositions with various organic cations are described in published patent application EP 961138.
Thin anisotropic films obtained using LLC systems based on sulfoderivatives of various organic dyes, including perylene dyes, have been characterized with respect to their properties and structures. In particular, the properties of films obtained using perylene dye based LLC systems have been studied. See I. K. Iverson, S. M. Casey, W. Seo, and S.-W. Tam-Chang, Controlling Molecular Orientation in Solid-Crystalline Phase, Langmuir 18(9), 3510–3516 (2002). All films were reported to possess a high degree of optical anisotropy.
The properties of thin anisotropic films obtained using an LLC system based on sulfoderivatives of organic dyes has been reported. See T. Fiske, L. Ignatov, P. Lazarev, V. Nazarov, M. Paukshto, Molecular Alignment in Crystal Polarizers and Retarders, Society for Information Display, Int. Symp. Digest of Technical Papers (Boston, Mass., May 19–24, 2002), p. 566–569. It was established that these films possess at least a partially crystalline structure. Optically anisotropic films can be obtained on substrates of glass, plastic, or any other material. The Violet dye used for the formation of these anisotropic films represents a blend of cis and trans isomers. See V. Nazarov, L. Ignatov, K. Kienskaya, Electronic Spectra of Aqueous Solutions and Films Made of Liquid Crystal Ink for Thin Film Polarizers, Mol. Mater. 14(2), 153–163 (2001). Possessing high optical characteristics, with a dichroic ratio reaching 25–30, these films can be used as polarizers. See Y. Bobrov, L. Blinov, L. Ignatov, G. King, V. Lazarev, Y.-D. Ma, V. Nazarov, E. Neburchilova, N. Ovchinnikova, S. Remizov, Environmental and Optical Testing of Optiva Thin Crystal Film™ Polarizers, Proceedings of the 10th SID Symposium “Advanced display technologies”, (Minsk, Republic of Belarus, Sep. 18–21, 2001), p. 23–30.
Methods for obtaining of such films, including those with high degree of crystallinity, are described in PCT Publication WO 02/063,660.
All of the aforementioned PTCA DBI sulfoderivatives are capable of forming LLC phases. Anisotropic films obtained using such LLC systems possess high optical characteristics and show good performance as polarizers.
However, one of the main disadvantages of the known water-soluble PTCA DBI sulfoderivatives is the difficulty of obtaining related anisotropic films possessing reproducible (from batch to batch and on different substrates in the same batch) and homogeneous (over the substrate surface) properties. The existing film application technologies require the process parameters (concentration, temperature, etc.) to be thoroughly selected and strictly followed. However, even in cases when all the conditions of film formation are strictly obeyed, random local violation of the coating structure is still possible. This is related to a certain probability of the formation of misorientation zones and micro defects as a result of non-uniform micro and macro crystallization processes in the course of solvent removal upon LLC system application onto a substrate surface. In addition, LLC systems based on the known dyes are characterized by increased probability of non-uniform thickness of the applied coating, which also decreases reproducibility of the film parameters. The aforementioned disadvantages complicate the formation of films possessing high optical characteristics, make the technology insufficiently reproducible, and require most of the technological parameters to be thoroughly selected and strictly followed in each stage, from application to drying.